This invention relates to a composition of kaolin clay pigments especially useful for coated wood free or coated free sheet printed paper. In particular, the invention relates to a kaolin coating pigment composition comprising particles where (a) greater than 96% by weight is less than 5 microns; (b) about 88% to 95% by weight is less than 2 microns; and (c) about 150i to 25% by weight is less than 0.25 micron. This pigment contains some kaolin books or stacks concentrated in the greater than 1 micron size fraction; has a shape factor which correlates to the average particle aspect ratio ranging between 5 and 14; and can be made down with water into a slurry having a minimum of 69% solids.
Kaolin clay pigments are obtained from kaolin. Kaolin, also known as china clay, is a type of rock that contains kaolin minerals. Kaolin minerals are hydrous alumina silicate clay minerals that include the minerals kaolinite, dickite, nacrite and halloysite. Primary kaolin deposits are formed in situ by weathering or hydrothermal alteration of feldspar or mica minerals contained in igneous or metamorphic rocks to kaolin minerals. Sedimentary kaolin deposits contain kaolin mineral-bearing sediments that were eroded and transported from a primary kaolin source rock, kaolin minerals resulting from the post-depositional alteration of feldspar- and mica-bearing sediments, or both. Commercially mined kaolins can contain some impurities such as clay-sized ferruginous or titaniferous impurities that impart an undesirable color to the kaolin clay. Some kaolin deposits contain impurities such as clay minerals of the smectite group that have an undesirable effect on the rheology of the kaolin. Most commercial kaolin deposits have coarse-grained sand to silt size impurities called xe2x80x9cgritxe2x80x9d that are generally above 45 microns. Grit particle impurities in a kaolin clay pigment used for paper coating may cause imperfections on the coated paper surface that resemble scratches or cause excessive wear of the paper coater blade by abrasion of metal surfaces in contact with the grit-bearing pigment.
The most common kaolin mineral is a naturally occurring hydrous alumina silicate known as kaolinite (Al2Si2O5 (OH)4). Kaolinite is the most common mineral in the kaolin clay widely used in the paper industry as fillers and/or coating pigments. Kaolin clays used for pigments are also called china clay or hydrous kaolin. Kaolinite particles occur over a range of sizes and a range of aspect ratios. The aspect ratio for kaolinite is defined as the diameter of a kaolin particle-divided by its thickness. Thus, a kaolin deposit mined commercially will generally not contain particles of a single size, such as, for example, particles all of which are 2 microns. Typically after minimum refining, a degritted kaolin will contain particles ranging in size from submicron or colloidal to particles 45 micrometers or larger.
Kaolin from different deposits, or even from different parts of the same deposit, can vary widely in the content of impurities, particle size distribution, as well as shape, or aspect ratio, of the kaolinite particles. In general, kaolinite particles smaller than about 1 micrometer occur as individual platelets. As the size of the kaolinite particles increase from about 1 micrometer, a larger proportion of the kaolinite particles occur as stacks or booklets of platelets as opposed to discrete individual platelets. Particle sizes of kaolin clay are conventionally determined by sedimentation using Stokes Law to convert settling rates to particle size distribution, and assume a spherical particle shape for the kaolin particles, hence, the use of the conventional term xe2x80x9cequivalent spherical diameter (e.s.d.)xe2x80x9d to designate particle size.
Kaolin clay pigments are widely used to coat and to fill paper products. It was formerly the practice to simply use relatively coarser size kaolins to fill papers and to employ finer size kaolins to coat paper. In paper, the coarser kaolin fillers functioned primarily as a pulp extender. When used to coat paper, the finer kaolin pigments improve brightness of the paper, provide a smooth, ink-receptive surface, and improve gloss with improved print quality and aesthetic appearance.
Kaolinite stacks or booklets can be mechanically delaminated. Delamination is generally known as the process of splitting kaolinite stacks or booklets, along a basal 001 cleavage or parting plane, into discrete platelets. One delaminating operation involves subjecting the naturally occurring kaolin stacks to shearing forces in an extruder, thereby reducing the kaolin stacks to discrete platelets. Another operation involves subjecting the naturally occurring kaolin stacks in an aqueous clay slurry to the cleaving or delaminating action of an attrition mill or a sand grinder. Reference may be made to U.S. Pat. No. 3,615,806 of Andrew Torock and Thomas F. Walsh for a thorough discussion of the process of delamination of kaolin clay. The kaolin pigments which have been delaminated can be used in paper coating to improve the opacity as well as enhance the smoothness of the paper surface. See for example, U.S. Pat. No. 3,171,718 to Gunn et al.
Kaolin clay can also be thermally structured through a calcination process which irreversibly converts the kaolinite into a material called xe2x80x9cmetakaolinitexe2x80x9d. Calcination causes the kaolin particles to stick or fuse together into porous aggregates and results in a pigment with higher light scattering caused by a higher index of refraction and a higher amount of light scattering surface than a kaolin which is not calcined.
Kaolin clay, hydrous kaolin, (Al2Si2O5(OH)4) is white in color, has a fine particle size, and is relatively chemically inert, and, in addition to its low cost, makes it an ideal paper filler. Calcined (anhydrous) kaolin is also available for use as a filler and can impart greater opacity to paper than the hydrous kaolin. However, calcined kaolin has the serious disadvantage of being more abrasive than hydrous kaolin clays. Hydrous-and calcined kaolin clays are used to coat paper. However, calcined kaolin is generally used as a minor blend component in a coating pigment formulation because of its higher cost, poor glossing properties, and higher abrasion.
The purpose of paper coating is to cover an irregular paper surface comprised of cellulose wood fiber with a pigment-binder formulation, that when dry, leaves a smooth and brighter surface ready for printing. It is common practice to use kaolin clay along with other mineral pigments, such as titanium dioxide and calcium carbonates, as a coating in a coating formulation comprising starch and/or latex.
U.S. Pat. No. 4,241,142 to Kaliski, et. al. and assigned to Engelhard Minerals and Chemicals Corporation, discloses a novel clay pigment used in the production of dull and matte-finished coated printing papers. The novel clay pigment comprises a mixture of a coarse-size fraction of naturally-occurring kaolin clay containing a substantial proportion of kaolinite booklets in the particle size range of 2 to 5 microns e.s.d., and mechanically delaminated kaolin platelets in the particle size range of 2 to 10 microns.
A paper coating pigment comprising mechanically delaminated kaolin particles is disclosed in U.S. Pat. No. 5,169,443 to Willis, et al. and assigned to Engelhard Corporation. This pigment possesses the opacification, smoothness and printability advantages of conventional delaminated kaolin pigments but have desirably low viscosity and gloss not characteristic of conventional delaminated kaolin pigment. This pigment is designed for gravure and offset printed light-weight coated groundwood paper.
U.S. Pat. No. 2,992,936 to Rowland and assigned to Georgia Kaolin Company discloses an undelaminated and defined kaolin clay pigment with 10% to 15% of particles below 0.3 microns that provides improved brightness, opacity and gloss on coated paper.
U.S. Pat. No. 5,085,707 to Bundy et al. and assigned to Georgia Kaolin Company discloses a delaminated and defined kaolin clay pigment and its blend with a delaminated, No. 1 coating and No. 2 coating clays to obtain superior opacity, brightness and gloss on coated paper. This disclosure describes a standard engineered kaolin pigment sold commercially today.
Some coating formulations involve a combination of different types of kaolin pigments, such as a certain percentage of calcined kaolin clay and a certain percentage of hydrous kaolin clay, which combination can result in xe2x80x9cpoorxe2x80x9d dispersion of particles in that the two types of kaolin clay pigments may aggregate, which may disrupt the coating structure, resulting in poor sheet optics, poor printability, or both. Additionally, many paper coating compositions comprise an abundance of fines, which tend to affect the optical properties of the sheet by improving gloss at the expense of sheet brightness for some grades and opacity.
Finally, kaolin clay pigments for coating are commonly shipped to paper mills as slurries by railroad tank cars, tanker trucks or in tank compartments on ships or barges. The shippable solids of a kaolin clay pigment slurry is dependent upon its fluidity at high solid levels both in terms of its low-shear viscosity that relates to how the slurry flows out of a tank and its high-shear viscosity that relates to how the slurry remains fluid as it passes through pumps and under the blade of the paper coating machine. Engineered pigments for coated freesheet applications such as Astra-Plus(trademark) generally has a shippable pigment-water slurry solids close to 65%. Delaminated kaolin coating clays typically have a shippable pigment-water slurry solids below 68%. Standard No. 1 and No. 2 coating clays typically have a shippable pigment-water slurry solids of 70% or higher. There is an economic incentive to ship kaolin clay pigments at the highest possible solids to reduce the cost of shipping water with the pigment. Also, paper coating formulations for the coated freesheet market generally have a restrictive water balance to maintain quality and cost without adding expensive thickening agents into the formulation. It is also commonly theorized that higher coating solids improves surface coverage, that is the ability for pigment particles to cover the network of wood fibers and pores that form the surface of the paper basesheet. Therefore, paper coaters prefer pigment slurries shipped at the highest possible solids.
There is, therefore, a need in the industry for a fluid, low viscosity coating pigment for paper, particularly for wood free or free sheet paper, which provides for a highly glossy surface with good opacity and sheet brightness.
The present invention has met this need. The present invention provides a paper coating composition where the pigments are preferably comprised of one type of mineral, that is kaolinite (Al2Si2O5(OH)4), as opposed to a combination of kaolinite and calcined kaolin containing metakaolinite, mullite or spinel phases. The coating composition of the invention involves a kaolin pigment which is engineered to contain kaolinite stacks or books in the coarse particle size fraction of the pigment (greater than 1 micron) which provides sheet fill for macro smoothness and fluidity at higher clay-water slurry solids, and platy kaolinite particles that occupy the coarse and fine particle size fraction which forms a micro-smooth paper surface for good printability. The engineered kaolin pigment in the composition of the invention also comprises a paucity of fine particles below 0.25 micrometers which helps to enhance optical properties of the paper by creating pores in the coating structure which are optimally sized to scatter light. In theory, the coating composition of the invention provides kaolin stacks to fill the valleys or gaps between the fibers to obtain sheet smoothness, a paucity of fines to provide good opacity and sheet brightness, and platy particles to provide for a high gloss surface in proportions that provide for a surprising fluid clay-water slurry at 70% solids. The coating composition of the invention is preferably applicable to wood free or free sheet paper where high solids pigment slurries are desired.
To provide for a fluid clay-water slurry at 70% solids, several properties in the pigment are balanced to maintain performance of the engineered pigment on the coated sheet and maximize fluidity of the slurry. The properties that influence fluidity of a clay-water slurry are 5 and 2 micron contents, 0.25 micron content and particle aspect ratio. The less than 5 micron and less than 2 micron contents of the pigment are maximized to prevent scratching and poor sheet gloss of the coated paper surface caused by too many coarse particles and minimized to optimize particle packing which is essential for fluidity. The less than 0.25 micron content of the pigment is maximized to optimize particle packing while set to minimize the presence of fines that would fill voids or pores between larger pigment particles and cause loss of light scatter and coating opacity. The average aspect ratio or shape factor is minimized on the coarse side of the particle size distribution, by the presence of kaolinite stack and books, in order to minimize particle-particle contacts in a slurry under shear and is not controlled on the fine side of the particle size distribution in order to contribute to the glossing properties of the clay on the coated paper sheet.
The particle size distribution of the kaolin clay pigment in the composition of the invention is characterized in that greater than 96% by weight of the particles is less than 5 microns of which about 5% to about 50% by weight or by volume of the greater than 1 micron fraction are kaolinite books or stacks; between about 88% to about 95% by weight of the particles is less than 2 microns; between about 65% to about 85% by weight is less than 1 micron; and between about 15% to about 25% by weight of the particles is less than 0.25 microns.
The invention uses the shape factor of the kaolin pigment as a correlation of the average aspect ratio of particles in the kaolin pigment. Kaolinite stacks have a low particle aspect ratio that is generally less than 1. Books of kaolinite platelets in crystallographic union that represent fragments of broken stacks can have a low particle aspect ratio in the 1 to 8 range. Individual kaolinite plates have a higher particle aspect ratio that is generally greater than 8 and potentially exceeds 20. A first embodiment of the invention involves a kaolin clay pigment that has a shape factor equal to or less than 10, and preferably between about 7 and 9, which represents a significant number of stacks and books, and some platelets. A second embodiment of the invention involves a kaolin clay pigment that has a shape factor between 10 and 14, and preferably 11 or 12, which represents some stacks and books with a majority of plates.
The engineered kaolin clay pigment of the invention forms a clay-water slurry about equal to or greater than 70% solids. When the pigment is incorporated in a coating, the composition gives a G. E. brightness ranging from between 88 and 94, and a Hunter b-value below 3.5.
It is, therefore, an object of the present invention to provide a kaolin clay pigment used as a component of a coating composition for wood free and free sheet paper grades comprised of an engineered kaolin clay having a shape factor being less than 10 or not greater than 14 that functionally provides good sheet optics, good printability, or both to the coated paper.
It is a further object of the present invention to provide a kaolin clay pigment used as a component of a coating composition for free sheet and wood free paper grades consisting of an engineered kaolin clay with a particle morphology of kaolinite stacks, books and plates being greater than 1 micron.
It is a further object of the invention to engineer a kaolin pigment by using a shape factor measurement and using the value as a correlation to obtain the desired particle size distribution for the kaolin pigment.